It is known that, in order to transfer a liquid from a reservoir to a usage station arranged at higher height, generally, pumping members are used, such as in particular submerged pumps. Known submerged pumps are generally provided with an external tubular casing provided with a plurality or radial openings for liquid inlet, normally referred to as suctioning openings, and with an outlet opening, referred to as discharge opening. There are accommodated inside the casing the pump operating members, constituted of a pumping unit and a motor unit generally of the electrical type, suitable to operate the liquid transfer from the suctioning openings to the discharge opening.
The pumping unit is usually constituted of one or more stages, each of which comprising a rotating member or impeller, suitable to be actuated in rotation by the motor unit shaft, and by stationary members suitable to convey the liquid from one stage to another, up to the discharge opening.
U.S. Pat. No. 7,290,984 for example illustrates a centrifugal pump with multiple stages, in which each stage is constituted of an impeller unit, a disc unit and a distributor unit, arranged around a motor shaft. The hub of the impeller is provided with floating sealing means, supported by the diffuser unit, suitable to reduce the liquid recirculation. A thrust ring is provided, arranged around the motor shaft and suitable to engage the hub of the impeller and the above mentioned impeller hub sealing means. Lastly, a friction coupling ring is provided, fixed against the disc unit and suitable to engage a corresponding surface of the impeller.
A problematic aspect of the electric pumps of the known type, and in particular of the submerged pumps, regards protecting the electric motor and the electric connections against the liquid transferred from the pumping member. To this aim, there is usually shaped in the electric pumps of the prior art, inside the casing, a special motor chamber suitable to be separated, by means of mechanical sealing means, both from the liquid present outside the casing, and from the liquid that passes through the pumping member.
Connected with such problem is the need, especially common for the submerged electric pumps, to check the operating members present inside the external tubular casing. As a matter of fact, this type of electric pumps is often used to operate on liquids that, though filtered, still contain dirt particles. In order to prevent the mechanical components from being worn by such particles, it is necessary to check and cyclically clean the mechanical parts in contact with the liquid. However, in the electric pumps of the known art it is difficult to combine the need for a perfect protection of the electric motor with the need to easily disassemble and reassemble the parts forming the casing to perform the maintenance operations of the electric pumps.
In an attempt to solve the above mentioned problems, different types of devices for fastening and connecting the parts constituting the electric pumps have been proposed. For example, U.S. Pat. No. 4,523,899 illustrates a submerged electric pump of the multistage type provided with two tubular portions which comprise respectively a motor unit and a pumping unit, coaxially connected through an elastic joint member. Such joint member is suitable to insulate the portion which contains the motor unit from the lubrication liquid.
Patent EP0774584 illustrates a different type of sealing connection means intended to be used in electric pumps of the submerged type. In particular, it is provided a device for mounting and for connecting the parts forming a pumping unit. Such pumping is provided with two connection units with screws arranged peripherally and in an alternating manner. The first connection unit connects by means of a sealing ring a first shell suitable to hold the stator member of the motor unit and a second shell suitable to hold the lubricating fluid. The second connection unit connects such sealing ring, a third shell of the pumping member, and an external cooling casing that surrounds the shell of the stator member.
However the proposed solutions do not allow to satisfactorily overcome the above mentioned drawbacks, in that they comprise expensive and complex fixing devices, that require long and complicated mounting and dismounting operations. In particular, such problems are more significant in the case of the electric pumps of relatively small dimensions, with diameters of the motor member comprised for example between 2 and 10 inches, for which it is more difficult to provide fixing solutions that facilitate the partial dismounting of the different operating parts.
Another problem unsolved by the electric pumps of the known art stands in the fixing of the axial position of the motor shaft. The aforesaid electric pumps comprise rolling means connected to the motor shaft and fixed to parts of the pump made through moulding. These elements have thus dimensional inaccuracies that do not enable to determine precisely the positioning of the motor shaft.